An optical switch has been intensively researched and developed, because the optical switch has been considered to be a key element in high speed optical communication and information processing systems for the future. One type of the optical switch is fabricated from a dielectric material such as LiNbO.sub.3, etc., and another type of the optical switch is fabricated from a semiconductor material such as GaAs, InP, etc., wherein the latter type has been expected to be used widely in the above systems, because it can be integrated with an optical device such as an optical amplifier, etc. and an electronic device such as an FET, etc., so that size can be small and the number of channels is increased in the systems. In this semiconductor optical switch, characteristics such as high speed operation, low power consumption operation, low voltge operation, low loss operation, feasibility for high integration, etc., are required to be applied to the systems.
Now, there have been fabricated and studied a total reflection type optical switch, in which a refractive index is changed in accordance with a band-filling effect or a free-carrier plasma effect by injecting current thereinto, a directional coupler type switch, in which a refractive index is changed in accordance with an electrooptic effect by applying an electric field thereto, and another total reflection type optical switch, in which a refractive index is changed in accordance with a shift of an exciton absorption peak by applying an electric field to a multi-quantum well therein.
Among these optical switches, the current injecting total reflection type has disadvantages in that operation speed is low, and electric power consumption is high, and the multi-quantum well total reflection type has a disadvantage in that a low loss structure is inherently difficult to be fabricated.
On the contray, the directional coupler type switch utilizing an electrooptic effect has advantages in that high speed operation, low electric power consumption, and low loss operation are realized, although a device length is longer as compared to the total reflection type.
In regard to this low loss characteristic, a GaAs/AlGaAs optical waveguide having a loss as low as 0.15 dB/cm at a wavelength of 1.52 .mu.m is described on pages 1628 to 1630 of "Appl. Phys, Lett. 50 (23), 8 June 1987," Such a low loss characteristics is realized in this optical waveguide, because bandgap wavelengths of GaAs and AlGaAS are positioned on a wavelength side which is sufficiently short as compared to 1.3 and 1.5 .mu.m bands. Furthermore, the optical waveguide has an advantage in that a refractive index change does not differ from that in the vicinity of a bandgap, even if an operation wavelength is separated from the bandgap, because the electrooptic effect has a less wavelength dependency. For these reasons, the GaAs/AlGaAS optical waveguide is highly expected to be a material for a directional coupler type optical switch at a long wavelength band.
Although a directional coupler optical switch is as long as several mm in a device length, this can be small by utilizing a recently developed minute processing technology such as Molecular Beam Epitaxy (MBE), Reactive Ion Beam Etching (RIBE), etc. In this point, a small-sized optical directional coupler switch, in which a length is made to be 980 .mu.m, that is, less than 1 mm by use of MBE and RIBE, is described on pages 1241 to 1243 of "Electronics Letters, 6th November 1986, Vol. 22, No. 23"
The small-sized optical directional coupler switch comprises an i-GaAs waveguide layer sandwiched by cladding layers of p-and n-AlGaAs and grown on a n.sup.+ -GaAS substrate of a (100) plane or a plane equivalent thereto, wherein the waveguide layer provides two parallel waveguides.
In operation, a switching operation is carried out by supplying an incident light of TE mode to one of the two waveguides. Here it is defined that a polarized waveguide light having an electric field component which is mainly parallel to layers of the switch is TE mode, while a polarized waveguide light having an electric field component which is mainly perpendicular to the layers is TM mode.
However, the small-sized optical directional coupler switch has a disadvantage in that a switching operation is not realized by an incident light of TM mode. Furthermore, a predetermined switching property is not obtained, where an incident light having a polarization changing in time is supplied to one of the two waveguides, even if a predetermined constant voltage is applied to the switch, because an output light power supplied from one of the two waveguides varies dependent on the change of the polarization.
In the past, a crystal orientation substrate of a (100) plane or a plane equivalent thereto has been studied for an optical semiconductor directional coupler switch utilizing electrooptic effect. However, crystal structures of GaAS and InP are isotropic for zincblende type structure. Therefore, a refractive index is changed in GaAs and InP only for a polarized light having an electric field component which is perpendicular to a direction of applied electric field, where a crystal orientation of (100) plane or a plane orthogonal thereto is utilized.